How Ntp Works - H3C S3100V2-52TP Configuration Manual

How NTP works

Figure 5 shows the basic workflow of NTP. Device A and Device B are connected over a network. They
have their own independent system clocks, which need to be automatically synchronized through NTP.
Assume that:
Prior to system clock synchronization between Device A and Device B, the clock of Device A is set
•
to 10:00:00 am while that of Device B is set to 1 1:00:00 am.
• Device B is used as the NTP time server (Device A synchronizes its clock to that of Device B).
It takes 1 second for an NTP message to travel from one device to the other.
•
Figure 5 Basic work flow of NTP
NTP message
Device A
1.
Device A
2.
Device A
3.
NTP message received at 10:00:03 am
Device A
4.
The process of system clock synchronization is as follows:
Device A sends Device B an NTP message, which is timestamped when it leaves Device A. The time
•
stamp is 10:00:00 am (T1).
When this NTP message arrives at Device B, it is timestamped by Device B. The timestamp is
•
1 1:00:01 am (T2).
When the NTP message leaves Device B, Device B timestamps it. The timestamp is 1 1:00:02 am
•
(T3).
When Device A receives the NTP message, the local time of Device A is 10:00:03 am (T4).
•
Up to now, Device A has sufficient information to calculate the following two important parameters:
The roundtrip delay of NTP message: Delay = (T4–T1) – (T3-T2) = 2 seconds.
•
• Time difference between Device A and Device B: Offset = ((T2-T1) + (T3-T4))/2 = 1 hour.
Based on these parameters, Device A can synchronize its own clock to the clock of Device B.
This is only a rough description of the work mechanism of NTP. For more information, see RFC 1305.
10:00:00 am
IP network
NTP message
IP network
NTP message 10:00:00 am
IP network
IP network
Device B
10:00:00 am 11:00:01 am
Device B
11:00:01 am 11:00:02 am
Device B
Device B
10